They are sirens from Equestria who were banished to the human world by Star Swirl the Bearded to keep them from using their dark magic to conquer Equestria.They are first seen in a café causing tension between the patrons, and Aria grumbles about the world's lack of satiating magic.
She works magic at the mic, helping the Dazzlings live up to their name.
Aside from flashbacks, Aria's true form is never physically shown in the film, but she and her fellow band members gain pony-like ears, extended ponytails, and translucent fin-like wings.
They also summon astral projections of their true siren forms from their pendants.
In musical terminology, the word "aria" refers to a long accompanied song for a solo voice, typically one in an opera; the word is used for the songs This Day Aria, Part 1 and This Day Aria, Part 2.
In the season seven episode Shadow Play - Part 2, Aria and her fellow sirens appear in flashback, creating discord among ponies with their singing, before they were defeated by the Pillars of Old Equestria and banished by Star Swirl the Bearded.